This invention relates to a magnetic memory, and more particularly, to a magnetic memory which can reduce power consumption by applying a magnetic field to a record layer of the magnetoresistance effect element efficiently by small write-in current.
Recently, there is a proposal of magnetic random access memory using a magnetic element exhibiting giant magnetoresistance effect as a solid magnetic storage device. Especially, magnetic memory using “ferromagnetic tunnel junction” is remarked as a magnetic element.
Ferromagnetic tunnel junction is mainly made of a three-layered film of first ferromagnetic layer/insulating film/second ferromagnetic layer, and a current flows, tunneling through the insulating film. In this case, the junction resistance value varies proportionally to the cosine of the relative angle between magnetization directions of the first and second ferromagnetic layers. Therefore, resistance value becomes minimum when the magnetization directions of the first and second ferromagnetic layers are parallel, and becomes maximum when they are anti-parallel. This is called tunneling magnetic resistance (TMR) effect. For example, in the literature, Appl. Phys. Lett., Vol. 77, p 283(2000), it is reported that changes of resistance value by TMR effect reaches as high as 49.7% at the room temperature.
In a magnetic memory including a ferromagnetic tunnel junction as a memory cell, magnetization of one of ferromagnetic layers is fixed as a “reference layer”, and the other ferromagnetic layer is used as a “recording layer”. In this cell, by assigning parallel and anti-parallel magnetic orientations of the reference layer and the recording layer to binary information “0” and “1”, information can be stored.
For writing information, magnetization of the recording layer is reversed by a magnetic field generated by supplying a current to a write line provided for the cell. In many cases, magnetization reversal is performed by passing write-in current simultaneously to two write-in wirings which cross each other without touching, and thereby applying a synthetic magnetic field in the direction which has a certain angle which is not zero to a magnetization easy axis of a memory layer.
Moreover, in the case of this write-in operation, an “asteroid curve” indicating the relation between the direction of an applied magnetic field and a reversal magnetic field is taken into consideration.
On the other hand, read out is performed by flowing a sense current through the ferromagnetic tunnel junction, and by detecting a resistance change by TMR effect. A number of such memory elements are aligned to form a large-capacity memory device.
Its actual configuration is made up by placing a switching transistor for each cell and combining peripheral circuits similarly to DRAM (dynamic random access memory), for example. There is also a proposal of a system incorporating ferromagnetic tunnel junctions in combination with diodes at crossing positions of word lines and bit lines (U.S. Pat. Nos. 5,640,343 and 5,650,958).
For higher integration of magnetic memory elements using ferromagnetic tunnel junctions as memory cells, the size of each memory cell becomes smaller, and the size of the ferromagnetic element forming the cell inevitably becomes smaller. There is the same situation in magnetic recording systems when the recording density is enhanced and the recording bit size is decreased.
In general, as the ferromagnetic element becomes smaller, its coercive force increases. Since the intensity of the coercive force gives criteria for judging the magnitude of the switching magnetic field required for reversal of magnetization, its increase directly means an increase of the switching magnetic field. Therefore, upon writing bit information, a larger current must be supplied to the write line, and it invites undesirable results such as an increase of power consumption, shortening the wiring lifetime, etc. Therefore, it is an important issue for practical application of high-integrated magnetic memory to reduce the coercive force of the ferromagnetic element used as the memory cell of magnetic memory.
In order to solve this problem, a magnetic memory element equipped with a thin film which becomes the circumference of write-in wiring from material which has a high magnetic permeability is proposed (the U.S. Pat. No. 5,659,499, the U.S. Pat. No. 5,956,267, the U.S. Pat. No. 5,940,319, and international patent application WO 00/10172).
In these elements, a magnetic flux generated by write-in current can be converged by thin film which has a high magnetic permeability in the circumference of write-in wiring. Therefore, a magnetic field generated at the time of writing can be strengthened, and bit information can be written in with smaller current as the result. At the same time, since a magnetic flux which leaks to the exterior of a high magnetic-permeability thin film can be reduced greatly, an effect which can reduce cross talk is also acquired.
However, in the case of the structure currently disclosed in the U.S. Pat. No. 5,659,499, a magnetic field cannot be uniformly applied over the magnetic whole record layer.
Moreover, in the case of the structure disclosed in the U.S. Pat. No. 5,956,267 and the U.S. Pat. No. 5,940,319, a magnetic field cannot be applied efficiently to the magnetic record layer, since a distance between the high magnetic-permeability thin film and the magnetic record layer is large and especially the distance becomes too large in the case where two or more magnetization pinned layers are provided in order to obtain a higher output.
On the other hand, in the case of structure disclosed in the international patent application WO 00/10172, it has structure where distance between the high magnetic-permeability thin film and the magnetic record layer becomes small, however, it is difficult to concentrate sufficient magnetic flux to the magnetic record layer.
Moreover, in any of these disclosures, it is not devised at all about a cross sectional shape of the write-in wiring.